The goal of the proposed work is to develop a method for quantitating the simultaneous contributions of carbohydrate, lipid and amino acids to oxidative metabolism in the slow, oxidative rate soleus muscle, and then to systematically evaluate the effects of changing substrate levels and the interaction of insulin with the direct substrate effects. The analysis will be based on a steady state model of labelled substrate use which is most sensitive to metabolic branch points and subcellular pool separation. The results are expected to complement studies measuring enzyme activities or intermediate concentrations and held our understanding of the dynamics of metabolic regulation.